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Skiff Rocker

Good morning everyone! Lately I've been looking at building a skiff. While reading up on the subject, some questions came up about rocker. In K. D. Jones book HOW TO BUILD A FLATION SKIFF Simple Steps Using Basic Tools, the rocker is determined by the amount of twist in the planks and is changed by adjusting the angle of the plank at the transom from vertical. The bottom plank does not have any curve cut into it. In Edwin Monk's book HOW TO BUILD WOOD BOATS With 16 Small-Boat Designs, plan for the 11'-6" Skiff plans show a curve cut into the bottom plank. This starts out an 11 1/2 inch width at the base of the stem reducing to 7 inch width at the transom. Both of these boats are built "Skiff Style" where the side planks are bent around frames (or spreader stick) and attached to the transom to get the shape.

The questions that come up are:

Does the raked stem on Monk's 11'-6" skiff reduce the amount of twist in the plank and require that the bottom plank be shaped?

Is one method for adding rocker better than the other? If so, why?

Is the amount of rocker added by twisting the planks as in the Flatiron skiff enough to make it a good rower?

Thanks in advance for your replies. I apologize if this is a trivial question, I'm just trying to understand what's going on in each of these designs.
Clay

Re: Skiff Rocker

We need more info, the drawings would be good. What sort of curve, round or S shaped?
For a rowing skiff the transom should be narrower than for a motor skiff and clear of the water. The rake of the stem will not affect the rocker forward, just the twist in the topside planks if the midship flare is the same. Basically more curve, more rocker.
I would advise making a cardboard model from each of the plans to see how they turn out.

It really is quite difficult to build an ugly wooden boat.
The power of the web: Anyone can post anything on the web
The weakness of the web: Anyone can post anything on the web.

Re: Skiff Rocker

You're looking at the marvelous ways that NA's have for a good boat that's buildable with labor and material efficiencies. It's kind of magic.

In the picture below, the unprepossessing flattie 'Pup' is the left most dink and the shot only shows you that there's a dink there. Pup was meticulously designed by Spaulding Dunbar in the early '50s. Both her small size and the availability of superlight outboards in those days meant that Pup was designed for oar power only. The curve of the bottom matters. I heard that Dunbar made many models and a couple of prototypes tinkering mostly with that curve and a bit with the subtle flare and the gentle reduction in beam from amidships to the transom. Once perfected, the many Chatham area builders who knocked these things out did not change anything. The boat just rows beautifully light or laden.

And in all that, Dunbar was designing for light plywood construction and was making the shape as efficient in material use as possible. There are many ways to establish the bottom shape and no one is "best" for any boat. Rather, how you match that curve to the bottom of the hull, how that chine runs, depends upon all the other dimensions.

So best advice, assuming whoever designed and made your plans knew what's what, try to avoid the assumption that you're smarter.

Re: Skiff Rocker

Alright, I’ll chime in but only because I love skiffs. Bear in mind, this is only my opinion based on having built more than a dozen skiffs.

I’ve only built a few on strongbacks. I built a Mississippi River yawl type skiff on one, and a sailing skiff to a drawn design. The other skiffs all came to life on the same set of sawhorses I used to build the most recent.

All other things being equal, flare adds rocker, and sheer, if the board is parallel edged. This is a neat trick to get some sheer in the gunwales of skin on frame kayaks, by the by. 15-20* flare will give a snazzy looking sheerline.

The problem is, straight flare (unchanging from end to end) and straight sided boards tend to make banana shaped boats. The bottom edge of the side planks are often eased in certain areas to reduce the amount of rocker, but also maybe to add rocker in an area where the planks’ angle approaches vertical. It’s all a balance and compromise. No one method is the best, because no one method can produce every result.

It sounds, to me, like the designers in the op are after slightly different boats, so use different techniques.

The shape of the bow and the planks there are a combination of the flare at the bow, and the desired look, as well as the material used. Ply and solid wood bend quite differently is certain situations, see?

The upper edge of planks (on single strake sides, or the upper edge of the uppper strake) are often shaped to affect the sheerline of the boat, regardless of what shape the flared sides give a naturally straight edge. Sheerline are fussy and personal things, as are boats.

What has been said regarding rocker need not be repeated. Motors no, oars yes. Lots of stuff in the boat needs more rocker, less stuff in the boat can use less rocker. Wider boats generally want a bit more rocker, too, for a given length.

Also, the amount of rocker is dependent on the job requirements. A pulling boat aimed at traveling in a straight line will want less rocker overall than a drift or pond boat, for example, which may need to spin in her own length.

“Three board” skiffs are easy to build, but hard to make look nice from all angles. Lapstrake sides help immensely. All those snazzy lines are great detractors from any minor quibbles that may otherwise be detected. And flat sided skiff are so easy to plank lapstrake...

I recently built one based on some panel shapes laid out by a modern designer, and a few of his mold shapes. I wanted some changes, so I made them, by eye, as I went. As a result, I won’t call my boat an “x” designed by “x”, because my I didn’t do exactly what he said to, so, I shan’t blame the deal on him! Haha. Plus, the changes made really are significant enough that this really isn’t the same boat. I futzed with sheer and a few of the mold shapes. And, yeah, I also mostly stole his method of making thwarts, too. This is for me and the kid to camp and play from. Sleep right in the bottom, row from a box/seat of single tholes.

An excellent modeling medium for small, simple boats is chipboard, which disguises itself as cereal and cracker boxes. It bends pretty much just like plywood does, so gives a good idea of what you’re gonna get.

Have fun.

Peace,
Robert

P.S. Building even simple chipboard models of these simple skiffs will give you a fine education in how the process works.

Re: Skiff Rocker

Originally Posted by Ian McColgin

So best advice, assuming whoever designed and made your plans knew what's what, try to avoid the assumption that you're smarter.

Ian, I in no way assume that I am smarter than the designer of the boats mentioned in my post and I made no mention of changing anyone's plans. I was asking about the method that each one of the designers used and if one has an advantage over the other. I did this to gain knowledge where I can better evaluate designs to see if they will fit my intended use. Forgive me for being an amateur and not knowing everything about boat design and building.

Re: Skiff Rocker

Originally Posted by ClayKorn

Ian, I in no way assume that I am smarter than the designer of the boats mentioned in my post and I made no mention of changing anyone's plans. I was asking about the method that each one of the designers used and if one has an advantage over the other. I did this to gain knowledge where I can better evaluate designs to see if they will fit my intended use. Forgive me for being an amateur and not knowing everything about boat design and building.

Clay

Just build a card model from both plans and compare the outcome.

It really is quite difficult to build an ugly wooden boat.
The power of the web: Anyone can post anything on the web
The weakness of the web: Anyone can post anything on the web.

Re: Skiff Rocker

Originally Posted by ClayKorn

Ian, I in no way assume that I am smarter than the designer of the boats mentioned in my post and I made no mention of changing anyone's plans. I was asking about the method that each one of the designers used and if one has an advantage over the other. I did this to gain knowledge where I can better evaluate designs to see if they will fit my intended use. Forgive me for being an amateur and not knowing everything about boat design and building.

Clay

Hi Clay,

Rocker is a tough one.
I have struggled with it over the years making skin-on-frame, sit-on-top, prototype racing boats.
I've tried about 14 different configurations and am currently (no pun intended) building a 30', zero rocker, stabilized-monohull.
Kind of like a miniature EARTHRACE but 500 pounds versus 70-tons.
Skinny: 18:1 l/b ratio
Row and "sail".

Ian's comment, I believe, was an attempt at helping you avoid the pitfalls of builder-design-creep.
I don't believe it was meant as a personal attack but rather addressed a human nature impulse to make better a design.
I see the unfortunate dynamic at work here.
Please, give Ian the benefit of doubt and trust that he means well and was actually generous in his advice.